One in three drivers don't trust the fuel-economy numbers printed on window stickers of new cars are accurate. Motorists have good reason to be skeptical.

A new study of thousands of vehicles found real-world gas mileage significantly varies from the estimates certified by the Environmental Protection Agency. For many motorists, the discrepancies actually work in their favor. AAA, which conducted the study, says more than 8 in 10 drivers report beating the EPA estimates. Overall, U.S. drivers report gas mileage that's 12 percent higher than window-sticker estimates.

"For years we've heard that drivers question whether the fuel economy rating for their vehicle is accurate," said John Nielsen, AAA's managing director of automotive engineering and repair. "It's encouraging to see real-world fuel economy that more closely aligns with, or even exceeds, automaker promises."

Although the numbers typically favor motorists, inaccurate information can still hurt carmakers and consumers. Gas mileage is a key factor for many car buyers. Depending on the current cost of fuel, it might be the most important shopping criteria. So underestimated fuel-economy numbers could lead consumers to bypass vehicles they would have otherwise considered. EPA officials did not return a request for comment Tuesday.

Owners of vehicles with diesel engines and manual transmissions appeared to be the biggest beneficiaries of discrepancies between estimated and actual fuel economy. Diesel owners reported 20-percent higher fuel economy than EPA ratings. Owners with cars equipped with manual transmissions enjoyed 17-percent higher real-world results.

Owners with turbocharged engines, on the other hand, were the biggest losers. Owners of turbocharged V6 engines reported fuel economy 9-percent lower than estimates, and owners of turbocharged four-cylinder engines reported fuel economy that was 4-percent lower than expected.

"If you just think about how those cars drive, a diesel has a ton of torque down low, so small throttle changes will get an immediate response," Nielsen told Autoblog. "And a turbo, conversely, does need to spool up. I can't help but wonder what role driving habits play in that."

AAA culled its data from an analysis of 37,000 records submitted to the EPA, and those records contained more than 8,400 vehicle make, model, and model-year combinations.

Fuel-economy numbers have often been a confusing issue for consumers. The EPA actually tests only about 10 to 15 percent of new vehicles on the road. Automakers largely self-certify gas-mileage numbers by testing pre-production prototypes of their own vehicles, and they send the results to the EPA for window stickers.

This process predictably leads to problems. In November, Hyundai and Kia agreed to pay a $100-million fine to settle an investigation into fuel-economy exaggerations in approximately 1.2 million vehicles. In June 2014, Ford lowered the fuel-economy estimates on six vehicles after customer complaints.

In conducting its analysis, AAA engineers identified a list of vehicles that were frequently reported as failing to achieve the EPA's mileage rating. Many of the Hyundai and Kia models that carried exaggerated fuel-economy estimates, since revised downward, were identified in the trend information. AAA engineers say three additional vehicles -- a 2014 full-size pickup, a 2014 large sedan and a 2012 medium-sized sedan -- also were flagged as frequently falling short of estimates.

But when the engineers conducted further tests on those unnamed models, they found the vehicles' fuel economy met EPA estimates. With no apparent mechanical shortcomings, they focused on driver behavior as a possible culprit for the poor numbers.

Fundamentally, consumers don't always understand that their driving habits play as large a role in fuel economy as vehicle shortcomings. AAA says its study found driving behavior and environmental conditions, rather than car troubles, are "likely responsible" for most fuel-economy variances. Nielsen said there's another study in the works that analyzes that issue. Results are scheduled to be released later this year.

To that end, the organization offers a few tips for drivers looking to stretch their fuel budget:

Accelerate gently: The harder you accelerate, the more fuel you consume. Maximize fuel efficiency by taking five seconds to accelerate from a stop to 15 miles per hour.

Avoid high speeds: Vehicles are most efficient when traveling between 30 and 50 miles per hour. Every five mph above 50 mph costs drivers an extra 19 cents per gallon of gas, according to estimates from the Department of Energy.

Maintain steady speed: Impossible on your commute, probably. But tests have shown that using cruise control on the highway can save fuel by avoiding speeding and slowing down.

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